In conventional wind turbine power production systems the energy from the wind is transferred mechanically, either directly or by a rotational speed-up gear to an electric generator.
The generator must rotate at a nominal speed to be able to deliver electricity to the grid or network connected to the power production system. If, during low wind speed conditions, the turbine is not supplying an appropriate level of mechanical torque to the system it will fail to deliver energy and instead the generator will act as an electric motor and the net will drive the generator and turbine through the mechanical gear.
On the other hand, if the wind is too strong the rotational frequency of the wind turbine rotor may become too high for the generator to operate properly or the mechanical apparatus could break down due to the strong forces.
Several solutions exist for overcoming the problems related to varying wind conditions. The most obvious solution is to stall and/or brake the turbine or pitch the turbine blades when the wind is too strong. Manual brakes and pitch control of the turbine blades are in use today, however, this solution may lower the efficiency of the system.
A well known solution from background art is the use of inverters to convert the output frequency of the electric generator to a desired frequency. The generator driven by the turbine will then be allowed to run at a variable rotational frequency depending on the wind speed. The use of inverters may be costly and may reduce the overall efficiency of the system.
It is known from background art that mechanical transmission systems based on planetary gears with variable gear ratio can be employed to maintain the generator rotational speed close to a desired value during varying wind conditions.
In U.S. patent application 2005/194787 and international patent application WO-2004/088132 a wind turbine where the transfer of energy from the turbine to the generator is mechanically gear driven is described. The gear ratio can be varied by varying the rotational speed and direction of the outer ring of the planetary gear. In these applications a hydrostatic transmission system is used for controlling the planetary gear.
It has been proposed in several publications to use a hydrostatic transmission system comprising a hydraulic pump and a hydraulic motor for transferring energy from the turbine to the generator. By employing a hydraulic pump and/or motor with variable displacement, it is possible to rapidly vary the gear ratio of the hydraulic system to maintain the desired generator speed under varying wind conditions.
Japanese patent application JP 11287178 by Tadashi, describes a hydraulic transmission system used for the transfer of energy from a wind turbine rotor to an electric generator where the generator speed is maintained by varying the displacement of the hydraulic motor in the hydrostatic transmission system.
Hydrostatic transmission systems allow more flexibility regarding the location of the components than mechanical transmissions.
The relocation of the generator away from the top portion of the tower in a wind turbine power production system removes a significant part of the weight from the top portion of the tower. Instead the generator may be arranged on the ground or in the lower part of the tower. Such an arrangement of the hydrostatic motor and the generator on the ground level will further ease the supervision and maintenance of these components, because they may be accessed at the ground level.
International patent application 94/19605A1 by Gelhard et al. describes a wind turbine power production system comprising a mast on which is mounted a propeller which drives a generator. The power at the propeller shaft is transmitted to the generator hydraulically. The propeller preferably drives a hydraulic pump which is connected by hydraulic lines to a hydraulic motor driving the generator. The hydraulic transmission makes it possible to locate the very heavy generator in a machinery house on the ground. This reduces the load on the mast and thus makes it possible to design the mast and its foundation to be lighter and cheaper.
U.S. Pat. No. 7,183,664 (McClintic) describes a wind turbine where a hydraulic pump pressurizes fluid and stores the fluid in a chamber in the support tower. Pressurized fluid is directed via a proportioning valve to a hydraulic motor which is coupled to an electric generator. The proportioning valve is controlled to maintain the rotational speed of the generator. At the top of the tower a hydraulic swivel couples the pump outlet to the high pressure chamber in the tower, and from a low pressure tank to the hydraulic pump. The low pressure fluid returns to the pump by using an inert pressurized gas in the low pressure tank.
DE 3025563 (Suzzi) describes a wind turbine with a closed loop hydrostatic transmission system comprising a hydraulic swivel. NO20041044 (Nikolaus)
Norwegian patent application NO22045083 (Bragstad) describes a swivel for transporting water under pressure from a water pump driven by a wind turbine rotor to a turbine for generation of electricity.
WO2006029633 (Andersen) describes a wind turbine comprising one or more pumps. The pumps are connected to a device for converting hydraulic pressure to a rotational movement to drive a generator. The pressure from the pump is controlled by a valve and a tank for pressure balancing. A device for compensation of rotational movement is used between the nacelle and the tower.
WO2007143902 and CN2911237 describes a swivel device.
It can be seen from WO2007053036 (Chapple et al) the energy transfer between a turbine rotor and an electric generator may be made more efficient by controlling the displacement of the hydraulic pump or motor based on one or more speed measurements, such as the wind speed.
In real-life hydraulic pumps and motors will always have a small leakage of fluid that has to be handled. For large installations the leakage can be considerably large, and has to be handled carefully.
A trend in the field of so-called alternative energy is that there is a demand for larger wind turbines with higher power. Currently 5 MW systems are being installed and 10 MW systems are under development. Especially for off-shore installations far away from inhabitated areas larger systems may be environmentally more acceptable and more cost effective. In this situation the weight and maintenance access of the components in the nacelle of the wind turbines is becoming a key issue. Considering that about 30% of the downtime for a conventional wind turbine is related to the mechanical gearbox, the weight of a 5 MW generator and the associated mechanical gear is typically 50 000 to 200 000 kg and that the centre of the turbine stretches 100 to 150 m above the ground or sea level, it is easy to understand that the deployment and maintenance of conventional systems with mechanical gears and generator in the nacelle is both costly and difficult.
Thus, there is a need for reducing the weight and the number of critical and heavy-weight components in the nacelle in wind turbine power production systems according to background art with a new and innovative solution that reduces the number of components in the nacelle and the need for deployment and maintenance of components in the nacelle. Further, a solution for handling the oil leakage occurring in pumps and motors is needed for such hydrostatic transmission systems to be successfully deployed in real-life systems. Efficient energy transfer from the wind turbine rotor to the generator during varying operational conditions represented by changing wind speeds and generator load should be considered.